The present invention relates to an HDTV system, and more particularly, to an NTSC interference detector for detecting an NTSC signal to thereby determine whether a post-comb filter is used or not.
FIGS. 1 and 2 illustrate the Grand Alliance transmission system which is America's HDTV system.
The transmission system, as shown in FIG. 1, comprises a video encoder 1 for compressing digital video signals R, G and B, an R-S coder & trellis coder 2 for Reed-Solomon coding and trellis coding the signal output from video encoder 1 for the purpose of easy error correction, a precoder 3 for precoding the signal output from R-S coder & trellis coder 2 and thereby canceling NTSC interference, a modulator & SAW filter 4 for forming the signal output from precoder 3 into a form of vestigial side band within 6 MHz, and a transmitter 5 for transmitting the signal output from modulator & SAW filter 4 through an RF channel.
The receiving system, as shown in FIG. 2, comprises an RF tuner & IF SAW filter 6 for selecting a channel and outputting an IF signal, a demodulator 7 for converting the signal output from RF tuner & IF SAW filter 6 into a baseband signal, a data clock recovery portion 8 for detecting a sync signal and clock from the signal output from demodulator 7, a post comb filter & controller 9 for detecting NTSC interference from the signal output from data clock recovery portion 8 and canceling the NTSC interference, a channel equalizer 10 for compensating for a distorted signal output from post comb filter & controller 9, an error correction & trellis decoder 11 for correcting the error of the signal output from channel equalizer 10 through R-S decoding and trellis decoding, a deformatter 12 for deformatting the signal output from error correction & trellis decoder 11, and a video decoder 13 for converting the signal output from deformatter 12 into the original digital video signals R, G and B.
The transmission system for transmitting a broadcasting signal may or may not use precoder 3 depending upon whether the NTSC broadcasting station is present or not. In the receiving system for receiving the broadcasting signal, the use of past comb filter 9 for performing the reverse function of precoder 3 should be determined based on whether or not precoder 3 is employed by the transmission system.
However, the use of precoder 3 cannot be perceived at the receiving port. If there is NTSC interference, precoder 3 is decided to be used and, post comb filter 9 is also employed. If there is no NTSC interference, precoder 3 is not used. In this case, post comb filter 9 is also not used and the signal output from data clock recovery portion 8 passes without change and is output to channel equalizer 10. This process is performed in post comb filter & controller 9.
FIG. 3 is a block diagram of a conventional NTSC interference detector for determining whether the post comb filter of FIG. 2 is used or not.
As shown in FIG. 3, the conventional NTSC interference detector comprises an NTSC removing filter 14 made up of a delay 141 and a subtractor 142 for removing the NTSC component from the received I-channel signal, an NTSC removing filter 15 made up of a delay 151 and subtractor 152 for removing an NTSC component from a data field sync reference pattern, a subtractor 16 for obtaining the difference between a received I-channel signal and the data field sync reference pattern, a subtractor 20 for obtaining the difference between the NTSC removed signal output from NTSC removing filter 14 and the signal output from NTSC removing filter 15, an absoluter 17 for taking the absolute value of the signal output from subtractor 16, an absoluter 21 for taking the absolute value of the signal output from subtractor 20, an integrator 18 for integrating the signal output from absoluter 17 for a predetermined time, an integrator 22 for integrating the signal output from absoluter 21 for a predetermined time, and a minimum energy detector 19 for comparing the signals output from integrators 18 and 22 and selecting a lowest-energy signal between the two signals, thereby outputting a control signal for controlling multiplexer 23 which then selects one of the signal passing through NTSC removing filter 14 and the received I-channel signal.
The operation of the conventional NTSC interference detector will be described with reference to FIGS. 4A, 4B and 4C. The RF spectrum of an HDTV signal to be transmitted is as shown in FIG. 4A. The RF spectrum of an NTSC signal to interfere is as shown in FIG. 4B. The frequency characteristic of NTSC removing filter 14 is shown in FIG. 4C.
Since a picture carrier, color sub-carrier, and audio carrier, all of the NTSC signal, are present around the null point, if the NTSC signal passes through NTSC removing filter 14, those components are canceled. Therefore, if the received I-channel signal passes through NTSC removing filter 14, the NTSC component is removed therefrom.
The received I-channel signal may be selected directly by multiplexer 23 and output without the NTSC interference being removed. The received I-channel signal may also be selected by multiplexer 23 through NTSC removing filter 14 composed of delay 141 and subtractor 142 and output with the NTSC interference removed.
Here, if there is NTSC interference, the signal output through NTSC removing filter 14 is selected by multiplexer 23. If there is no NTSC interference, the received I-channel signal which has not passed through NTSC removing filter 14 is selected and output by multiplexer 23. In order to control the selection of multiplexer 23, there must be an NTSC interference detector for detecting whether the NTSC interference is present or not.
In this case, a data field sync reference pattern is present in the receiver itself so that the signal in which a data field sync signal and data field sync reference pattern have passed NTSC removing filters 14 and 15 out of the received I-channel signal are compared with the data field sync signal and data field sync reference pattern of the I-channel signal, thereby outputting a control signal for controlling multiplexer 23 according to the comparison result.
This process will now be described in greater detail.
Out of the received I-channel signal, the data field sync signal is input to subtractor 16 so that the difference between the data field sync signal and the data field sync reference pattern is calculated. This value indicates the magnitude of NTSC interference. The signal output from subtractor 16 is converted into a positive value in absoluter 17 and integrated for a predetermined period through integrator 18, for instance, until the data field sync reference pattern and data field sync signal are input. The integrated signal is then output to minimum energy detector 19.
Out of the received I-channel signal, the NTSC component of the data field sync signal is canceled through NTSC removing filter 14. The NTSC component of the data field sync reference pattern is canceled through NTSC removing filter 15.
The signal output from NTSC removing filter 14 is input to subtractor 20 and subtracted from the signal output from NTSC removing filter 15. The signal output from subtractor 20 is converted into a positive value in absoluter 21 and integrated through integrator 22 for a predetermined period, for instance, until the data field sync reference pattern and data field sync signal are input. The integrated signal is then output to minimum energy detector 19.
Minimum energy detector 19 compares the signals output from integrators 18 and 22 and outputs a control signal for controlling multiplexer 23 so that multiplexer 23 selects the one of lower energy and a signal corresponding to the lower energy.
If the signal output from integrator 18 has lower energy, minimum energy detector 19 decides that there is no NTSC interference and outputs the control signal so that multiplexer 23 selects and outputs the received I-channel signal. If the signal output from integrator 22 has lower energy, minimum energy detector 19 decides that there is NTSC interference and outputs the control signal so that multiplexer 23 selects and outputs the signal output from NTSC removing filter 14.
However, the conventional NTSC interference detector must detect the presence or absence of NTSC interference based only on the data field sync reference pattern and data field sync signal. This results in a high possibility that the detection of NTSC interference may be inaccurate.